1. Field of the Invention
The present invention relates to a piezoelectric resonator, a piezoelectric resonator component, and a method of making the same.
2. Description of the Related Art
Conventionally, piezoelectric resonator components making use of a piezoelectric transducer as a resonator for obtaining oscillation frequencies are known. An example of a piezoelectric transducer includes a piezoelectric substrate with a pair of electrodes on one side and the opposite side thereof. The piezoelectric resonator component has the piezoelectric transducer fixed to a capacity element providing two load capacities which constitute an oscillation circuit, with one side of the former perpendicular to the thickness direction facing one side of the latter perpendicular to the thickness direction. The piezoelectric resonator component also has an input electrode, an output electrode, and a grounding electrode electrically and mechanically connected to the piezoelectric transducer or the capacity element via the respective connecting conductors and is sealed with a sealing cap.
Such piezoelectric resonator components are disclosed in JP-A-60-123120, JP-A-1-236715, JP-A-8-237066, and JP-A-10-135215.
Known piezoelectric resonator components utilizing a thickness extensional vibration mode include those using a fundamental wave vibration mode and those using a harmonic wave vibration mode, especially a third harmonic wave vibration mode.
Energy-trapping resonator components are typical of piezoelectric resonator components using a third harmonic vibration mode. Since the piezoelectric substrate used in an energy-trapping resonator has parts that do not vibrate, the resonator can be fixed at these parts to provide components which hardly suffer from deterioration of characteristics and find wide applications.
Piezoelectric resonator components of thickness extensional fundamental vibration mode utilizing fundamental wave vibrations exhibit high resonance characteristics represented by a high Qmax value. However, they hardly have non-vibrating parts unlike the energy-trapping resonators. In a small-sized component, in particular, the whole piezoelectric substrate vibrates, making it difficult to support and fix the substrate stably.
On the other hand, a piezoelectric substrate has been mounted on a dielectric substrate via joints of conductive paste dried to cure. In the case of piezoelectric resonator components using fundamental wave vibrations, the adhesive strength of the joints tends to be instable due to variation of adhesive area caused by conductive paste viscosity variation in mounting or due to seeping. Variation or reduction in joint adhesive strength is liable to lead to deterioration of characteristics due to vibration energy suppression, deterioration of resonance characteristics due to insufficient control of spurious vibrations, and oscillation defects such as instable skipping of necessary oscillation.
As another example, conventional piezoelectric resonator components have a piezoelectric transducer fixed on one side of a substrate with a connecting conductor. An input electrode, an output electrode, and a grounding electrode are connected to the substrate both electrically and mechanically via the respective connecting conductors, and subsequently the piezoelectric transducer fixed on the substrate is sealed with a cap.
In such a structure wherein a piezoelectric transducer is fixed to one side of a substrate via a connecting conductor, differences in linear expansion coefficient among the connecting conductor, the substrate and the piezoelectric transducer tend to produce thermal stress, which can develop cracks in the connecting conductors. This results in reduced reliability of interconnectivity of the components. To solve this problem, JP-A-8-288291 proposes using a connecting conductor including a resin ball coated with a solder film thereby to relax the thermal stress caused by the difference in linear expansion coefficient between the conductor, the substrate and the piezoelectric transducer.
However, because the proposed connecting conductor has a resin ball as a nucleus, it is likely that the adhesive area between the connecting conductor and the substrate or the piezoelectric transducer varies and the adhesive strength therebetween tend to be reduced. There is another problem that some components in the resin ball tend to seep and the adhesive strength therebetween tend to be reduced.
JP-A-11-340776 discloses a connecting conductor including a nucleus made of Cu, Ag, carbon, glass, ceramics, resins, etc. having an electrically conductive film formed thereon.
The above-described prior arts, however, do not teach techniques for relaxing the thermal stress arising from the difference of linear expansion coefficient because the materials proposed for forming the nucleus show linear expansion coefficients largely different from those of the piezoelectric element or the substrate.